The purpose of this study is to determine whether long term treatment of psoriasis with oral 8-methoxypsoralen (8-MOP) plus longwave ultraviolet light (UVA) may induce temporary or permanent damage of the dermal connective tissue. Patients will be treated with 8-MOP and UVA (PUVA) for 1 to 3 years, and skin biopsies will be taken before and after 1, 2, 3 years of treatment. The skin specimens will be studied by light and electron microscopy to determine possible alterations in collagen, elastic tissue, blood vessels and fibroblasts. Chemical analysis will be performed to determine (a) collagen types (b) ratio of Type I/Type III collagen, (c) soluble and insoluble collagen (d) glycosaminoglycans and (e) elastic tissue. Fibroblasts will be obtained from skin explants before and after 1 to 3 years of treatment with PUVA. "In vitro" metabolic studies will be performed to determined rate of collagen synthesis using radiolabeled precursors, collagen types and glycosaminoglycans. Fibroblasts from normal skin will also be used as controls for this study.